This application claims priority under 35 USC xc2xa7119 to Japanese Patent Application No. 2000-329129 filed on Oct. 27, 2000, the entire contents of which being herein incorporated by reference.
1. Field of the Invention
The present invention generally relates to optical writing units which perform optical writing, methods for driving the optical writing unit, image forming apparatuses such as printers, copiers, facsimiles, etc., which employ the optical writing unit, and inspecting apparatuses which inspect the optical writing unit. In particular, the present invention relates to optical writing units capable of suppressing density unevenness in an image formed by the image forming apparatus.
2. Discussion of the Background
Recently, based on the minimization of a digital image output instrument such as a digital copier, a printer, a digital facsimile, etc., an optical writing unit for digital writing use has been necessitated to be minimized. Such digital writing methods can be roughly categorized in to two at the moment. One is a light scanning system that performs scanning a light flux irradiated from a light source such as a semiconductor laser or similar devices using a light deflecting device and forms a light spot by means of a scan imaging lens. The other is a solid writing system that includes and uses an imaging element array so as to form a light spot of a light flux irradiated from a light emitting element array such as one of an light emitting diode (LED) array and an organic EL array made by LEDs aligned.
In the former light scanning system, since the light deflector scans a light, an optical path length becomes longer. In contrast, in the latter solid writing system, since the optical path can extremely be short, there exists an advantage such as compact optical writing unit.
On the other hand, in the solid writing system that includes the light emitting element array having a plurality of light emitting elements and imaging element array, unevenness of a light emitting quantity of the plurality of light emitting elements and that of a shape of the imaging element array may create unevenness of a light spot on an image carrier (e.g. photo-conductive member). Such unevenness can be related to rigidity, a position, and a spot radius. Owing to the unevenness, unevenness of density arises in an image output by an image forming apparatus that employs such an optical writing unit as an exposure unit. As a result, a fine image is hardly obtained.
To obtain a fine image by suppressing the unevenness of the density, a conventional optical writing unit proposes to extend corrections such as any one of light quantity constant correction that enables an emission light irradiated from each light emitting element to the PC member to have a constant value, and spot radius constant correction that enables an optical spot formed on the PC member to have a constant radius at given threshold. For example, Japanese Patent Application Laid Open Nos. 2-62257, 4-305667, and 11-227254 propose such light quantity constant correction and spot radius constant correction, and are known as prior arts.
Among these, the first prior art intends to uniformize an LED light emission quantity by controlling a driving time period for each LED based upon unevenness of a generated light value per each LED dot. The above-described second prior art intends to suppress unevenness of focal depth of a lens array and that of an print density caused by difficulty in packaging of an LED chip with a light quantity adjusted LED head that enables a spot width of an quantity of light to be constant at a given threshold. In addition, it is described in the above-described third prior art that a characteristic point in a light emission intensity distribution of a light emitting element is measured (detected), and light quantity correction data used for energy supply to the light emitting element is determined based upon the characteristic point. Also described is that assumed light quantity correction data is determined based upon unevenness of the light quantity and is corrected based upon the characteristic point, so that the light quantity correction data is determined. In addition, as the characteristic point, changes in a peak position, a peak value, and light emission radius are exemplified.
The light quantity constant correction is a correcting method for measuring an emitted light quantity irradiated from each light emitting element to a PC member with a light quantity measurement device, and changes a current amount supplied to each light emitting element, thereby setting a prescribed supply current amount enabling the emitted light quantity to be constant. The current supply amount is generally controlled by four bits of light quantity correction data, and the light emitting quantity is set at a precision of a few percentages error even admitting that the light quantity is constant in the present circumstances. On the other hand, the spot radius constant correction is a correction method for measuring a light spot radius formed on the PC member with a spot radius measurement device, and changing and setting a prescribed amount of a current that is supplied to each light emitting element and thereby enabling the spot radius to be constant. Since, as same in the above stated light quantity constant correction, the current supply amount is again controlled by four bits, there exists a limit on a controllable amount even admitting that the spot light radius is constant.
In addition, the third prior-art proposes that a light spot radius (Wi) of each light emitting element is measured. Then, it is determined if the light spot radiuses (Wi) represent upward convexity in a graph (not shown) when the light quantity constant correction is only performed using assumed light quantity correction data. The assumed light quantity correction data input to the light emitting element of No. (i) is corrected if the upward convexity appears.
However, the below-described problems generally exist in such a system.
First, assumed light quantity correction data is corrected only in a section that meets the above-described determination. Accordingly, optimization can not be performed over the entire valid image region. Specifically, another section not meeting the determination remains a condition made by the light quantity constant correction.
Second, according to the assumed light quantity correction data correcting system, the assumed light quantity correction data is corrected in accordance with unevenness (Wbi) from the average value (Wave) of (Wi). As a result, it simply performs the spot radius constant correction.
Accordingly, such a proposal shows the light quantity constant correction at a section and the spot radius constant correction at another section, and simply makes combination of light quantity and spot radius constant corrections. In addition, even admitting being constant, there exists a limitation on a controllable value as in the above-described light quantity and spot radius constant corrections.
Thus, according to these light quantity and spot radius constant corrections, each of a light quantity and a spot radius is only converged to a resolution level (i.e., 4 bits) of a current supply amount as correction data. As a result, since it is intended to mostly approximate a prescribed target value, a value obtained after performing one of these constant corrections for each light emitting element should vary at around the prescribed value.
Accordingly, the light quantity or the spot radius of each light emitting element is independently set, and remaining light emitting elements are not taken into account.
Improving a limit of resolution level of correction data can be achieved, for example, from conventional 4 bits to either 6 or 8 bits, to approximate the prescribed value. However, the improvement in the resolution limit causes an increase in data values, and requires increase in data transfer speed, thereby resulting in a cost increase.
Accordingly, an object of the present invention is to address and resolve the above and other problems and provide a new reference voltage generation circuit. The above and other objects are achieved according to the present invention by providing a novel image forming apparatus that includes a light emitting array composed of a plurality of light emitting elements that emits light beams, and an imaging element array that performs imaging with the light beam on a prescribed plane. A three-dimensional exposure light intensity distribution may be obtained per a light beam on the prescribed plane. A prescribed attribute may be obtained from the three dimensional exposure intensity distribution. A comparison value may be obtained from a unit of successive two or more attributes. The light emitting elements may be controlled so as to emit a light beam having a prescribed quantity that enable all of the comparison values for the entire exposure valid region to fall within a prescribed range.
In yet another embodiment, the attribute is an exposure width at a prescribed threshold in a cross section of the three-dimensional light intensity distribution. The cross section may be directed in either a light emitting element aligning direction or its orthogonal direction.
In yet another embodiment, the attribute is either an exposure width of a two dimensional light intensity distribution or an exposure area obtained by accumulating light intensities of the three dimensional light intensity distribution in either a light emitting element aligning direction or its orthogonal direction.
In yet another embodiment, the comparison value is either an inclination of an approximation linear line or a unit exposure area average obtained from a plurality of attributes.
In yet another embodiment, the imaging element array extends over the entire valid image region and is integrally molded.
In yet another embodiment, a positional relation adjusting device may be provided so as to adjust a positional relation between the light emitting element array and the imaging element array so that all of the comparison values for the entire exposure valid region to fall within a prescribed range.
In yet another embodiment, the positional relation adjustment is performed by displacing the imaging element array toward a defocus position.
In yet another embodiment, a computing and processing device determines a light emitting quantity, and each of the light emitting elements is driven based upon the light emitting quantity.
In yet another embodiment, the computing and processing device may find an attribute from a correlation of a correction value, i.e., a light emitting quantity, and its resultant attribute per a light emitting element.
In yet another embodiment, the computing and processing device may determine a range of attributes that the next light emitting element must take based upon one or more preceding known attributes of light emitting elements.
In yet another embodiment, the computing and processing device may determine the light emitting quantity of each of the light emitting elements based upon a correction value.
In yet another embodiment, a prescribed light emitting quantity that enables all of the comparison values to fall within the prescribed range is determined and set to each of the light emitting elements.
In yet another embodiment, an inspecting apparatus may be provided so as to inspect an optical writing unit or an imaging element array by inspecting if all of the comparison values fall within the prescribed range.